The subject of the present invention is a device for low-frequency communication by magnetic coupling between emission by magnetic field and a reception antenna.
In a more particular manner, the emission antenna can be disposed on a vehicle and the reception antenna in an identification member such as a tag or a plate of the credit card kind, thus allowing identification between said member and the vehicle and hence, thereafter, the exploitation of any desired control signal such as the unlocking of the doors or the switching on of the headlights or else the remote starting of the engine, when it is necessary to heat it in advance as is the case in cold countries.
It is known, in particular through the patent application JP-62-148044 held by the xe2x80x9cNISSAN MOTOR COMPANYxe2x80x9d, to dispose in the door of a vehicle two loop antennas oriented at 90xc2x0 to one another so as to obtain a magnetic field in two directions in space, thereby making it possible to have on the tag merely a reduced antenna which can have the dimension of a credit card.
The drawback of this device is that as the emission magnetic field emits in two dimensions only, it follows that, in certain positions of the receiver tag, the latter does not receive the signal, so that there is no communication between the car and the tag.
The first objective of the present invention is to eliminate this drawback by creating an omnidirectional emission magnetic field by the association of three coils according to the three axes in space.
The second objective of the present invention relates to the support devices for such three associated coils.
The invention therefore relates to a device for low-frequency communication by magnetic coupling of the type comprising a magnetic field emitter and a receiver placed in an identification member, one of either the emitter or the receiver being furnished with a loop antenna, characterized in that the other of either the emitter or the receiver is constituted by the association of three coils wound around three substantially perpendicular axes defining a trihedral so as to obtain an omnidirectional magnetic field by supplying said coils with currents of like frequency.
Advantageously, the omnidirectional magnetic field is generated by supplying the three coils with currents which are out of phase by about 60xc2x0 or 120xc2x0.
The three coils can be wound on one another on a common core for the three coils. The three coils of the emitter can be disposed on one and the same spherical core; this core can also be a parallelepiped around the six faces of which are wound the three coils; in one variant, this core forms a parallelepipedal lamina of small thickness, one of the coils being wound breadthwise across the core, in the vicinity of its middle part, the second coil being wound around the two longest large faces of the core and the third being wound on the small-thickness edge of the core.
Provision may also be made for the three coils to be wound on two separate cores; one of the cores has, for example, an approximately cubic shape and the other has, for example, the shape of a lamina of relatively small thickness, one of the coils being wound on the edge of the lamina of small-thickness, the other two being crossed at 90xc2x0 on the core of approximately cubic shape.
According to a preferred embodiment, the magnetic field emitter is constituted by the association of the three coils and is incorporated into a vehicle, and the receiver consisting of the loop antenna is incorporated into the identification member; as a variant, the magnetic field emitter, consisting of a loop antenna, is incorporated into a vehicle and the receiver constituted by the association of the three coils is incorporated into the identification member; the identification member and its antenna can be substantially planar.
Advantageously, the core(s) consists (consist) of a magnetic material, but it (they) could also consist of a material which is transparent from the magnetic point of view. The core(s) comprises (comprise), preferably, recesses for positioning and housing the coils of the emitter.